1. Field of the Invention
The present invention generally relates to a wavelength monitor that is adapted to measure a wavelength of a light as a measurement object, for example, a wavelength of a leaser beam emitted in single-mode emission. More specifically, the present invention relates to a wavelength monitor that is adapted to measure a stable-and-noise-reduced interference signal.
Priority is claimed on Japanese Patent Applications No. 2005-186185, filed Jun. 27, 2005 and No. 2005-317265, filed Oct. 31, 2005, the contents of which are incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the precut application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
A variety of light emitting devices are used in the fields of optical communication and optical measurement. Typical examples of the light emitting devices may include, but are not limited to, Distributed Feedback Laser Diodes (DFB-LD), Distributed Bragg Reflector Laser Diodes (DBR-LD), and External-Cavity Tunable Laser Diodes using a diffraction grating.
The Distributed Feedback Laser Diodes and the Distributed Bragg Reflector Laser Diodes have long-term drifts of emission wavelength. The External-Cavity Tunable Laser Diodes have thermally unstable wavelengths. Highly accurate and precise measurement and monitoring of the wavelength of the light we necessary for using the light emitting device exhibiting the single mode emission in the fields of optical communication and optical measurement.
Typical examples of the wavelength measuring apparatus may include, but are not limited to, a wavelength monitor with a diffraction grating, and another wavelength monitor that causes an interference of measured lights. Typical examples of the wavelength monitor using interference signals of the measured lights may include, but are not limited to, a wavelength monitor that uses an interference filter, and another wavelength monitor that measures two interference signals, both of which are different in phase by 90 degrees. The two interference signals may be so called as A-phase interference signal and B-phase interference signal.
Japanese Unexamined Patent Application, First Publication No. 10-253452 discloses a configuration of a conventional wavelength monitor. FIG. 13 is a block diagram illustrating a configuration of the conventional wavelength monitor. A beam of measured light to be measured by the wavelength monitor is incident into a cut filter 50. The cut filter 50 allows a selective transmission of the measured light in a predetermined range of wavelength.
The measured light is transmitted through the cut filter 50. The transmitted light is then incident into an interference filter 51. The interference filter 51 has a continuous variation in transmittable wavelength of the measured light over incident positions. A slide adjusting mechanism 52 is configured to mechanically slide the interference filter 51 by a small distance in a direction parallel to an X-axis. Sliding the interference filter 51 causes a continuous variation in wavelength of the light that is transmitted through the interference filter 51.
A photodiode 53 is configured to receive the transmitted light that has been transmitted through the interference filter 51. Another photodiode 54 is configured to receive a reflected light that has been reflected by the interference filter 51. A power ratio calculating unit 55 includes IV converter circuits 55a and 55b, a subtracter 55c, an adder 55d, and a divider 55e. The power ratio calculating unit 55 receives output signals from the photodiodes 53 and 54. The power ratio calculating unit 55 calculates a ratio of power between the photodiodes 53 and 54.
The IV converter circuits 55a and 55b are configured to receive outputs from the photodiodes 53 and 54 and to convert the outputs into voltage signals, respectively. The subtracter 55c is configured to receive the voltage signals from the IV converter circuits 55a and 55b and to perform a subtraction between the voltage signals. The adder 55d is configured to receive the voltage signals from the IV converter circuits 55a and 55b and to perform an addition of the voltage signals. The divider 55e is configured to receive results of operations from the subtracter 55c and the adder 55d and to divide the results of operations thereby normalizing an output ratio. The signal processing unit 56 is configured to receive the output ratio from the divider 55e and to calculate a wavelength of the measured light from the output ratio. In case of the wavelength monitor shown in FIG. 13, a measurable wavelength range and a measurable wavelength accuracy depend on wavelength characteristics of the interference filter 51.
Japanese Unexamined Patent Applications, First Publications No. 2000-234959 and No. 2002-214049 disclose other configurations of conventional wavelength monitors. FIG. 14 is a block diagram illustrating another configuration of the conventional wavelength monitor. An interferometer such as a Michelson interferometer is used to measure two interference signals differing in phase by 90 degrees, for example, A-phase and B-phase interference signals so as to measure a wavelength of the measured light.
In FIG. 14, an input optical fiber 60 transmits a beam of measured light and emits the measured light to a space. A lens 61 converts the measured light into a parallel beam of measured light, wherein the measure light has been emitted from the input optical fiber 60. A half mirror 62 performs as a first beam splitter. The half mirror divides the parallel beam of measured light into divided beams of measured light. The half mirror also couples the divided beams of measured light into a parallel beam of interference light. A first reflector 63 reflects a first one of the divided beams of measured light toward the half mirror 62. A second reflector 64 has a reflecting surface that has a step which dimension is d=λ0/8. The second reflector 64 reflects a second one of the divided beams of measured light toward the half mirror 62. The first and second reflectors 63 and 64 are placed so that a reflecting surface of each of the first and second reflectors 63 and 64 is vertical to an optical path of each of the divided beams of measured light, into which the measured light has been divided by the half mirror 62. The divided beams of measured light are transmitted on optical axes toward the first and second reflectors. Then, the divided beams of measured light are then reflected by the first and second reflectors 63 and 64. The reflected beams of measured light are then transmitted on the above optical axes toward the half mirror 62.
A reflecting prism 65 performs as a second beam splitter. The reflecting prism 65 divides the interference light beam into two divided beams of interference light. The reflecting prism 65 is placed so that a top-edge of the reflecting prism 65 is aligned to the step on the optical plane of the second reflector 64. The step on the optical plane provides a λ0/4 optical path difference. The first photodiode 66 receives a first one of the two divided beams of interference light from the reflecting prism 65. The second photodiode 67 receives a second one of the two divided beams of interference light from the reflecting prism 65. The signal processing unit 68 calculates a wavelength of the measured light with reference to outputs from the first and second photodiodes 66 and 67.
The following descriptions will be directed to operations of the above-described device.
The measured light is emitted from a light emission edge of the input optical fiber 60 toward a space. The emitted measured light is converted into the parallel beam of measured light by the lens 61. The parallel beam of measured light is incident into the half mirror 62. The parallel beam of measured light is divided into two divided beams of measured light by the half mirror 62. The two divided bears of measured light are transmitted to the first and second reflectors 63 and 64.
The first and second reflectors 63 and 64 reflect the two divided beams of measured light, into which the parallel beam of measured light is divided by the half mirror 62. The second reflector 64 has the reflecting surface that has the step which dimension is d=λ0/8. The step causes the optical path difference of λ0/4 between first and second half portions of the second one of the divided beams of measured light. λ0 is the wavelength, Preferably, the wavelength λ0 can be set at a center wavelength of the measured wavelength range. The wavelength λ0 can, for examples be set at 1550 nm for optical communication.
The reflected parallel beams of measured light that have been reflected by the first and second reflectors 63 and 64 are then incident into the half mirror 62. The reflected parallel beams of measured light are then coupled with each other to generate a parallel beam of interference light. The parallel beam of interference light is irradiated onto the reflecting prism 65 so that the parallel beam of interference light is divided by the top-edge into two divided beams of interference light. The two divided beams of interference light are different in phase by 90 degrees. The two divided beams of interference light are then incident into the first and second photodiodes 66 and 67. The two divided beams of interference light are converted into current signals by the first and second photodiodes 66 and 67. The current signals correspond to intensities or optical powers of the two divided beams of interference light. The current signals are supplied to the signal processing unit 68.
The signal processing unit 68 compares the intensities of light that have been supplied from the first and second photodiodes 66 and 67. The signal processing unit 68 outputs wavelength-related data. A variation of optical intensity over wavelengths obtained by the Michelson interferometer is given by the following equation (1).I=[1+cos[2π×ΔL/λ]]/2  (1)where I is the normalized intensity of light that is received by each of the fast and second photodiodes 66 and 67, λ is the wavelength of the measured light, ΔL is the optical path difference of the Michelson interferometer. One cycle of the variation of the optical intensity is so called to as a free spectral range (FSR). If the optical path difference is large, the free spectral range is small.
The second reflector 64 has the reflecting surface that has the step which dimension is d=λ0/8. The step causes the optical path difference of λ0/4 between first and second half portions of the second one of the divided beams of measured light. As a result, two periodical interference signals, for example, A-phase interference signal and B-phase interference signal differing in phase by π/2 are obtained The signal processing unit 68 calculates the variation of the wavelength of the measured light and confirms whether the wavelength increases or decreases.
Japans Unexamined Patent Application, First Publication No. 10-339668 discloses still another configuration of the conventional wavelength monitor. FIG. 15 is a block diagram illustrating still another configuration of the conventional wavelength monitor. A lease beam of measured light is emitted from an input optical fiber 70. The emitted measured light is twitted through a lens 71. The lens 71 converts the emitted measured light into a parallel beam of measured light. The parallel beam of measured light is transmitted through a polarizer 72. The polarizer 72 polarizes the parallel beam of measured light. The parallel beam of measured beam is then transmitted to a half minor 73. The half minor 73 divides the parallel beam of measured light into divided beams of measured light. A first one of the divided beams of measured light is received by a photodiode (PD) 74.
A second one of the divided beams of measured light is incident into a birefringent delay plate 75. The birefringent delay plate 75 has a fast axis and a slow axis. A combination of the fast axis and the slow axis causes a delay of λ/8 that corresponds to a phase shift of π/4 of polarized light having first and second polarizations. For example, the birefringent delay plate 75 causes a phase shift of the s-polarized light relative to the p-polarized light. The divided beam of phase-shifted light is then transmitted to a polarizing beam splitter 76. The polarizing beam splitter 76 splits the divided beam of phase-shifted measured light into a first beam of p-polarized light and a second beam of s-polarized light. The first beam of p-polarized light is transmitted to and received by a photodiode 77. The second beam of p-polarized light is transmitted to and received by a photodiode 78.
Outputs of the photodiodes 74, 77, and 78 are supplied to a signal processing unit 79. The signal processing unit 79 calculates a wavelength of the measured light. The measured light emitted from the input optical fiber 70 has a variation of optical power over times. An offset due to the optical power variation is corrected by the output from the photodiode 74.
FIG. 16 is a view illustrating relationship between wavelength and intensity of each of the s-polarized light and the p-polarized light to describe the principle of measuring the wavelength by the conventional wavelength monitor shown in FIG. 15. The horizontal axis represents the wavelength. The vertical axis represents the normalized optical power. An offset of the photodiodes 77 and 78 is corrected and normalized, thereby obtaining periodic interference signals differing in phase by 90 degrees from each other, for example, the A-phase interference signal and the B-phase interference signal.
The conventional monitors are so configured that the parallel beam of spatial light is incident into various optical elements such as the cut filter 50, the interference filter 51, the half mirror 62 and 73, the first reflector 63, the second reflector 64, the reflecting prism 65, the polarizer 72, the birefringent delay plate 75, and the polarizing beam splitter 76. This configuration allows a frequent appearance of multiple beam interference. The multiple beam interference superimposes desired multiple interference noise on the output signals from the photodiodes 53, 54, 66, 67, 74, 77, and 78, thereby causing a deterioration of the wavelength-measuring accuracy.
Each of the optical elements is formed of an independent optical part. Using a number of the optical parts makes it difficult to align the optical axis and also increases the manufacturing processes. This makes it difficult to reduce the size of the equipments and increases the cost and reduces the reliability.